Flexography is a type of relief printing that uses flexible sheets of photopolymer to transfer an image onto a substrate. In the flexography process, photopolymer sheet is exposed to light through a negative or mask that blocks selected portions of the photopolymer from the light. The portions of the photopolymer that are exposed to ultra-violet light crosslink. The non-crosslinked photopolymer is soluble in many organic solvents, whereas the crosslinked photopolymer is harder and much more resistant to chemical solvent attack. By “washing” the selectively-exposed photopolymer sheet in a solvent bath, the unexposed portions of the plate are removed, a process known as developing. The resulting developed plate will contain a raised relief image in those areas where ultra-violet light passed through the negative. The flexible relief plate is wrapped around a cylinder on a printing press, and used to transfer ink onto a substrate, such as paper, film, bags, etc.
While many organic solvents are capable of dissolving non-crosslinked material from the plate, only a few are considered good developer solvents. This is because most solvents that dissolve the non-crosslinked material also absorb into the crosslinked portions of the plate, causing these areas to soften and swell, thus changing the shape of the plate, and swelling encourages erosion of the relief image by the brush in the developer tank. This erosion results in the final image on the plate becoming different from the target image on the negative. Hence printing quality suffers. The ideal developer solvent would dissolve and remove the non-crosslinked portions of the plate while at the same time not softening or swelling the crosslinked portions.
Terpene hydrocarbons such as d-limonene have been used in developer solvents (see, i.e., U.S. Pat. Nos. 4,806,452 and 4,847,182). Terpene hydrocarbons have intense odors, are moderate skin and eye irritants, and have flash points below 141° F., making the waste solvent mixture “hazardous” by RCRA guidelines. This leads to increase disposal cost and high regulatory compliance costs. Additionally, d-limonene is unable to develop plates containing nitrile rubber (U.S. Pat. No. 5,077,177). Mixed aromatic solvents have been used in combination with butanol and 2-ethyl butanol (U.S. Pat. No. 5,312,719). However, these mixed aromatic solvents are relatively toxic and distinctly odoriferous, with low flash points (<141° F.).
Takagi et al. teaches the use of a three-part solvent blend containing mixed aromatic solvents to dissolve the photopolymer, an alcohol to remove the protective “anti-tack resin layer” or cover layer, and isobutyl isobutyrate as an odor masking agent (see, i.e., U.S. Pat. No. 5,578,420). The developer solvent contains 25-70% of mixed aromatic solvents such as Solvesso 150 that have high odor intensity and moderate toxicity. These solvents are so odoriferous that an odor masking agent (isobutyl isobutyrate) is required. Isobutyl isobutyrate has a flash point of only 99° F., classifying it as a flammable liquid. The addition of significant amounts of this solvent to the blend reduces the flash point of the mixture to below 141° F., causing the waste solvent to be classified as a RCRA hazardous waste. Additionally, the isobutyl isobutyrate itself also has a strong, unpleasant odor.
Despite a variety of organic solvents known to develop photopolymer plates, an odorless developing solvent that is easily recyclable has remained elusive. Thus, there remains a need for developing solvents with low odor intensity comprising non-hazardous components.